题目十六：合并两个排序的链表
题目描述：
输入两个单调递增的链表，输出两个链表合成后的链表，当然我们需要合成后的链表满足单调不减规则。
解题思路：
递归版本
public ListNode Merge(ListNode list1,ListNode list2) {
if (list1 == null) {
return list2;
}
if (list2 == null) {
return list1;
}
if(list1.val <= list2.val){
list1.next = Merge(list1.next, list2);
return list1;
}else{
list2.next = Merge(list1, list2.next);
return list2;
}
}
非递归版本
public ListNode Merge(ListNode list1,ListNode list2) {
if (list1 == null) {
return list2;
}
if (list2 == null) {
return list1;
}
ListNode mergeHead = null;
ListNode current = null;
while (list1 != null && list2 != null) {
if (list1.val <= list2.val) {
if (mergeHead == null) {
mergeHead = current = list1;
} else {
current.next = list1;
current = current.next;
}
list1 = list1.next;
} else {
if (mergeHead == null) {
mergeHead = current = list2;
} else {
current.next = list2;
current = current.next;
}
list2 = list2.next;
}
}
if (list1 == null) {
current.next = list2;
} else if (list2 == null) {
current.next = list1;
}
return mergeHead;
}
